Fine-scale Patchiness of Burns in a Mesic Eucalypt Savanna Differs with Fire Season and Sorghum Abundance

Abstract: Canopy tree populations in the eucalypt savannas of northern Australia consistently show bottlenecks in the transition of young trees to the canopy layer. This is most likely due to frequent fires, where young trees are caught in a 'fire trap', suffer topkill and regenerate from underground tissues year after year. Little is known of fire behaviour at the spatial scale of individual juvenile trees. This paper presents field measurements of small-scale, 10 cm (microsite), burn patterns and patch sizes following set fires in the early dry season, late dry season, and wet season. Fire treatments were repeated in plots dominated by a native annual grass (Sorghum) and in others with little or none of this grass. Both late dry season and wet season fires left few or no microsites unburnt, regardless of understorey type. With early dry season fire, however, understorey type made a difference: 47% of the microsites were left unburnt in plots with little or no Sorghum compared to only 5% in Sorghum-dominated plots. Further, the early dry season fire pattern was very patchy, resulting in both large and small areas of burnt vegetation. In Sorghum-dominated plots, more than half the patches were greater or equal 10 m in size and only a third were < 3 m in size, whereas in plots with little or no Sorghum, 99% of burnt patches were < 10 m in size, and 41% were < 0.5 m in size. Thus, only a regime of early dry season fire with little or no Sorghum in the understorey creates potential 'windows of escape' for juvenile trees. The study has implications for management of savanna trees by fire.